1) Field of the Invention
The invention relates to a plunger for feeding encapsulating material to a mould cavity. The invention also relates to a device for encapsulating electronic components mounted on a carrier. In addition, the invention also relates to a method for sealing a fitting of a plunger on a plunger housing, which plunger is adapted to feed encapsulating material under pressure to a mould cavity.
2) Description of the Related Art
In the encapsulation of electronic components, more particularly the encapsulation of semiconductors mounted on a carrier (lead frame), use is made of the so-called “transfer moulding process”. The carrier with electronic components is herein clamped between two mould parts such that mould cavities are defined around the components for encapsulating. Liquid encapsulating material is then introduced into these mould cavities and, after at least partial curing thereof, the mould parts are moved apart and the carrier with encapsulated electronic components is removed. The feed of encapsulating material takes place by means of one or more plungers with which pressure can be exerted on a supply of encapsulating material provided for this purpose. These plungers are displaceable in a housing into which the not yet liquid encapsulating material is also carried. The plunger exerts a pressure on the encapsulating material which is heated simultaneously and/or beforehand, as a result of which the encapsulating material becomes liquid. As a response to the pressure applied by the plunger, the liquid encapsulating material flows to the mould cavity and, under the correct processing conditions, fills it completely with encapsulating material. In the displacing of encapsulating material with a plunger the fitting of the plunger on the plunger housing is critical; uncontrollable quantities of encapsulating material must be prevented from penetrating through this fitting. In an existing solution for regulating the fitting of the plunger on the plunger housing a peripheral groove is recessed into the cylinder casing of the plunger. Encapsulating material collects in this groove which then cures and thus functions as seal in a subsequent production step. This is described for instance in U.S. Pat. No. 6,200,504. It has been found in practice that such a seal made from encapsulating material functions well initially, but that after a time there is a danger of the plunger jamming in the housing. This means that the plunger engages on the plunger housing in (too) close-fitting manner via the seal made from encapsulating material.
The present invention has for its object to provide an improved plunger, encapsulating device and method whereby the advantages of the above-described prior art can be retained and the chance of the plunger jamming in the plunger housing can be reduced.